1. Field of the Invention
The present invention relates to an endoscope system that is adapted to produce an image visualizing information about oxygen saturation of blood hemoglobin. The present invention also relates to a processor and an image producing method for the endoscope system.
2. Description of Related Art
In recent medical field, endoscopes are widely used for diagnoses and treatments. To inspect an interior of a test subject body through the endoscope, white or broadband light is ordinary projected into the test subject body for illumination, but also narrowband light of limited wavelength ranges come to be more frequently used as illumination light for the sake of making particular organs such as blood vessels conspicuous in subsequent endoscopic images displayed on a monitor screen.
Beside the endoscopic inspection using the special narrowband light, it has also been practiced in the art to derive information about vascular functions such as oxygen saturation of blood hemoglobin (percentage of oxygenated hemoglobin in blood vessels) or blood vessel depth from image signals captured by the endoscope, and visualize the derived information in the endoscopic image. For example, Japanese Patent No. 2648494 discloses allocating different colors to different oxygen saturation levels and producing a false-color oxygen saturation image on the basis of the allocated colors.
Japanese Patent No. 3315188 discloses modifying color tones of an endoscopic image according to the oxygen saturation levels, wherein the endoscopic image is captured in a three color frame-sequential fashion using a rotary color filter. Therefore the captured image generally reproduces ordinary colors of the subject. However, a red filter section of the rotary color filter has a wavelength band that is shifted to a range in which light absorbance of oxygenated hemoglobin remarkably differs from that of reduced hemoglobin. Thereby, particularly the hue of red of the endoscopic image will change according to the oxygen saturation level.
The false-color oxygen saturation image suggested in the former-mentioned prior art, however, shows totally different colors from natural or ordinary colors of the living body which is generally tinged with red. As a result, it is difficult to diagnostically inspect changes in mucosal properties. The latter-mentioned prior art, on the other hand, does not have such problem as the former prior art, since only the color tone or the tinge of red of an ordinary color image will change with the change in oxygen saturation. However, because the pixel levels of the red image signal depend not only on the oxygen saturation but also on the blood volume (sum of oxygenated hemoglobin and reduced hemoglobin) in the mucous, it is hard to scale the magnitude of oxygen saturation by the red hue gradation, discriminately from the magnitude of blood volume.